A large number of various apparatuses are known for examining or testing elongated textile test subjects such as card slivers, roving yarns, yarns or fabrics for example. Depending on their application, they can be categorized into the two classes of laboratory test (offline) and testing during the production process (online). The apparatuses make use of the various known sensor principles, of which the capacitive measuring principle is of special interest in this case, wherein a measuring capacitor is typically designed as a planar plate capacitor and comprises a through-opening for the test subject. The measuring capacitor is part of an LC oscillator, so that an electric alternating voltage is applied to the measuring capacitor upon excitation of the LC oscillator. The through-opening is thus subjected to an alternating electrical field. The test subject is moved through the plate capacitor and is subjected to the alternating field. An electric output signal of the plate capacitor is detected. Dielectric properties of the test subject are determined from the output signal in an evaluation circuit. Changes in the parameters of the test subject such as mass per unit of length and/or material composition are determined from the dielectric properties. A capacitive yarn or sliver sensor is described for example in GB-638,365 A.
In order to enable the performance of precise measurements which are not influenced by external influences such as air temperature or air humidity, a compensation method is frequently applied. For this purpose, the apparatus comprises a reference capacitor in addition to the actual measuring capacitor. It can be formed by adding a third capacitor plate arranged parallel to the two measuring capacitor plates, with the three capacitor plates being switched together into one capacitive measuring circuit. Examples for measuring circuits and suitable evaluation circuits for their output signals can be found in the specifications EP-0′924′513 A1, WO-2006/105676 A1 and WO-2007/115416 A1.
The measuring circuit should supply an output signal of the value zero when an alternating voltage is applied without test material. Due to the various imperfections in real electric components, it is not sufficient in practice to design the measuring circuit symmetrically in order to obtain a zero signal without the test material. Each measuring circuit needs to be balanced individually for symmetrization. Symmetric balancing occurs in production by the manufacturer and optionally during maintenance by a service technician. For this purpose, the capacitance of at least one capacitance trimmer connected in parallel to the measuring circuit is usually changed. Trimming occurs manually with a suitable tool, e.g. a screwdriver. The known method of laser trimming is applied alternatively. In any case, the apparatus needs to be opened for the balancing. Such a manual balancing is laborious, time-consuming and expensive.
DE-10'2005′006′853 discloses a measurement system with a sensor element which comprises a conductor structure forming a first oscillating circuit. A second oscillating circuit is coupled with the first oscillating circuit and contains a changeable component, so that its resonant frequency is adjustable. Since the two oscillating circuits form a coupled system, the resonance behavior determined by the first oscillating circuit can be influenced and thus shifted towards desired values. As a result, deviations in the capacitive or inductive components of the conductor structure which may optionally occur during the production or mounting of the sensor elements can thus be compensated.
GB-963,258 A shows a capacitive yarn clearer with a measuring capacitor through which the yarn passes. The voltage applied to the measuring capacitor is tapped and used for triggering the yarn cut. A variable resistor is switched in series with the measuring capacitor and the ends of the series circuit thus formed are connected to an AC voltage generator. The sensitivity of the yarn clearer can be set by changing the resistance.
U.S. Pat. No. 3,768,006 A shows apparatus and method for the capacitive measurement of the water content in an oil/water emulsion which flows in a pipeline. The outside wall of the pipe forms the outer grounded electrode of the measuring capacitor and an element arranged in the middle of the pipe forms the inner electrode. A reference capacitor of known capacity is connected in series with the measuring capacitor. An AC voltage generator generates an alternating voltage which is applied to the two capacitors. The voltage via the reference capacitor is measured and is a measure for the water content. The voltage over the measuring capacitor filled with the measuring subject is kept constant with a control circuit which controls the output amplitude of the AC voltage generator.
U.S. Pat. No. 3,684,953 A deals with an apparatus and method for the quantitative determination of a property of a dielectric test subject, especially its relative humidity content. The test subject is introduced into a measuring capacitor. And electric alternating signal is applied to the measuring capacitor, which signal is generated by an oscillator with an amplifier connected in series. A signal tapped from the measuring capacitor is demodulated and is compared with the alternating signal which is also demodulated and is applied to the measuring capacitor, in that quotient of the two signals is formed. The humidity content of the test subject is derived from the comparison. In order to maintain the dynamics range of the demodulators, the amplification of the amplifier is controlled by the demodulated output signal of the measuring capacitor. A variable balancing capacitor which is connected parallel to the measuring capacitor is provided for the zero balancing without test subject.
A device for the capacitive quality control of textile threads is known from U.S. Pat. No. 4,843,879 A. It contains a double-capacitor arrangement with a measuring capacitor and a reference capacitor. The double-capacitor arrangement is built into an electric circuit. The circuit contains an oscillator for applying two alternating voltages with opposite phases to the two outer capacitor electrodes of the double-capacitor arrangement. A signal amplifier and a balancing capacitor are disposed in each branch between the oscillator and the respective electrode. The balancing capacitors are used for balancing the output signal of the double-capacitor arrangement to the value of zero without any test subject.